1. Field of the Invention
The present invention relates generally to printed circuit boards (PCBs) and, more specifically, the present invention relates to a method an apparatus for a vented blind via in pad structure of a printed circuit board.
2. Background Information
A PCB typically includes a number of insulation and metal layers selectively patterned to provide metal interconnect lines (referred to herein as xe2x80x9ctracesxe2x80x9d), and a plurality of electronic components mounted on one or more surfaces of the PCB and functionally interconnected through the traces. The routing traces typically carry signals that are transmitted between the electronic components mounted on the PCB. Some PCBs have multiple layers of routing traces to accommodate all of the interconnections.
Traces located within different layers are typically connected electrically by vias formed in the board. A via can be made by making a hole through some or all layers of a PCB and then coating or plating the interior hole surface with an electrically conductive material. A via that connects all layers of the PCB, including the outer layers, is called a xe2x80x9cthrough via.xe2x80x9d A via that connects one or more inner layers to an outer layer is a xe2x80x9cblind via.xe2x80x9d
In order to fabricate PCBs in which electrical components are mounted in higher densities, a via in pad structure is often used. FIG. 1A is a perspective view diagram illustrating a prior art blind via in pad. A blind via 12 is configured in pad 10. The pad is formed on a multilayer PCB 16. Blind via 12 has an opening at pad 10 and extends into PCB 16. The walls of the blind via 12 are an electrically conductive material, such as copper. A solder mask (not shown) surrounds the pad 10.
One of the conventional ways of mounting electrical components on a PCB is called surface mount technology (SMT). SMT components have terminations or leads (generally referred to as xe2x80x9ccontactsxe2x80x9d) that are soldered directly to the surface of the PCB. The solder joint forms the physical and electrical connection between the component and the PCB. One conventional type of SMT component utilizes a ball grid array (BGA) to connect to the PCB. A BGA component has a plurality of solder balls on one surface, each of which represents an electrical contact.
The electrical contacts of an SMT component, such as a BGA component, are coupled to corresponding metallized mounting or bonding pads (also referred to as xe2x80x9clandsxe2x80x9d) on the surface of the PCB. Ordinarily one pad is dedicated to one SMT electrical contact.
Prior to mounting the SMT component on a PCB, the PCB is selectively coated with solder paste, using a mask (also referred to in the art as a stencil) that permits solder paste to coat just the pads. To mount an SMT component to a PCB, the component is carefully positioned or xe2x80x9cregisteredxe2x80x9d over the PCB so that its electrical contacts are aligned with the corresponding pads. Finally, the entire package is heated to a temperature that melts the solder paste (e.g., reflow soldering), to form a solder joint that is an electrical and physical connection.
During the heating process, one or more gas pockets can form within the soldier joint. Such gas pockets can be formed by expanding air bubbles trapped within the solder paste and/or blind via (also referred to as xe2x80x9coutgassingxe2x80x9d). FIG. 1B is a cross-sectional view diagram illustrating a prior art blind via in pad structure coupled to an electrical component. Blind via 12 is configured in pad 10. The blind via 12 extends into multilayer PCB 16 and is electrically coupled to an inner conductive layer 14. During the heating process to affix contact 20 of electrical component 22 to pad 10 by way of solder joint 24, a gas pocket 26 was formed by trapped air within the blind via 12. A solder mask (not shown) surrounds the pad 10.
Such a gas pocket can cause the solder joint to expand to the point where it touches an adjacent solder joint and, thus, create a short circuit. In the case of BGA components, this phenomenon is referred to as xe2x80x9cBGA bridging.xe2x80x9d The gas pocket may also cause the solder joint to crack and break resulting in an open circuit between the electrical component and the PCB. Additionally, a solder joint with a gas pocket may experience long term reliability issues due to repeated heating and cooling during its operative life.